1. Field of the Invention
The present invention relates generally to automatic focusing systems for a camera and in particular, to an automatic focusing system which can analyze a sequence of movements of a distance-setting or focusing member from an initial position to a focal position.
2. Description of the Related Art
A conventional automatic focusing system for a camera typically includes an objective lens focusing mechanism mounted to a shutter unit as shown in FIG. 6. A drive member 1 is rotatably mounted to a base plate 2 of the shutter unit and has a sector gear 1a operatively connected through an idler gear 3 to a stepper motor 4. The stepper motor 4 is rotatable in both directions according to a predetermined program to drive the drive member 1 for focusing, exposure and returning purposes. An operating cam 1b extends radially and outwardly from its outer periphery and cooperates with an operating lever 5 to open and close a shutter (not shown).
A distance-setting member 6 has a drive portion 6a designed to move an objective lens (not shown) to a focal position and is mounted to the base plate 2 of the shutter unit, like the drive member 1. The distance-setting member 6 also has a sector gear 6b in meshing engagement with a pinion 7a which is, in turn, integrated with a ratchet wheel 7. A spring 6c is arranged to rotate the distance-setting member 6 in a clockwise direction together with the drive member 1. An engaging lever or pawl 8 is pivotally mounted to the base plate 2 by a pin and biased by a spring 8a to rotate in a clockwise direction. The pawl 8 has an iron piece 9a which is attracted to an electromagnet 9. When the electromagnet 9 (also referred to as an AF magnet) is rendered nonmagnetized, then the pawl 8 is rotated in a clockwise direction to allow its free end to come into engagement with teeth of the ratchet wheel 7. This holds the distance-setting member 6 in a focal position.
A pinion 11a is fixedly mounted to one end of the stepper motor 4 and is operatively connected to the drive member 1. When the stepper motor 4 is rotated in a reverse direction, the pinion 11a causes the ratchet wheel 7 to rotate in a clockwise direction via the drive member 1, a pin on the cam 1b, and the distance-setting member 6. The pawl 8 is then moved out of engagement with the teeth of the ratchet wheel 7, the free end of which is held in a position away from the teeth of the ratchet wheel 7 upon energization of the electromagnet 9.
In a focusing mode of operation, the stepper motor 4 is rotated in a clockwise direction to drive the drive member 1 through the pinion 11a. The distance-setting member 6 is then rotated in a clockwise direction under the action of the spring 6c. A scanning lever (not shown) has a luminous element which is operative to provide a light beam upon the rotation of the distance-setting member 6. Such a light beam is reflected from the subject and is then received by a light receiving element (not shown). A focus signal is correspondingly developed so as to visually display distance information within the field of view of a camera viewfinder. Also, the electromagnet 9 is rendered nonmagnetized so as to allow the pawl 8 to rotate in a clockwise direction. This causes the pawl 8 to come into meshing engagement with the ratchet wheel 7 and thus restrains rotation of the distance-setting member 6. As a result, the objective lens is held in its focal position.
Upon further rotation of the stepper motor 4, the drive member 1 is rotated in a clockwise direction to cause the cam 1b to rotate the pin 5a of the operating lever 5 in a counterclockwise direction. The stepper motor 4 no longer controllably drives the distance-setting member 6 and is, in turn, active to open and close a shutter for exposure purposes. It will be noted that the stepper motor 4 can be rotated in a clockwise or counterclockwise direction in response to phase pulses from a stepper motor drive circuit 4a.
The distance-setting member 6, when held in a starting position, is constantly urged by the spring 6c to rotate in a clockwise direction. At this time, a home switch Hs, as a means for identifying the position of the distance-setting member 6, is held in an open state. This home switch Hs is closed when the distance-setting member 6 is moved from a starting position to a focal position. This indicates that the distance-setting member 6 is properly moved in a sequential manner.
In such a conventional automatic focusing system, the home switch Hs is mounted to the pin 6d of the distance-setting member 6. With this arrangement, the stepper motor 4 is rotated in a reverse direction so as to rotate the ratchet wheel 7 in a clockwise direction. The pawl 8 is then moved out of engagement with the teeth of the ratchet wheel 7. The free end of the pawl 8 is thus held in a position away from the teeth of the ratchet wheel 7. The distance-setting member 6 is released at the time of release of a first tooth of the ratchet wheel and then is free to rotate in a reverse direction through the pinion 7a to thereby close the home switch Hs. A minimum stroke is required for such rotation of the distance-setting member 6. Normally, the teeth of the ratchet wheel 7 correspond to 30 to 40 steps. However, such a minimum stroke of the ratchet wheel 7 requires eight pulses and occupies 20% of the teeth of the ratchet wheel 7. Thus, this stroke can not be used for focusing purposes.
The teeth of the ratchet wheel 7 and the free end of the pawl 8 are precisely finished in a manner to avoid improper engagement therebetween. If the first tooth of the ratchet wheel 7 is not properly engaged with the pawl 8, the distance-setting member 6 is not moved properly.